Method for receiving synchronizing signals for direct communication between terminals in wireless communication system, and device for same

ABSTRACT

A method of receiving a synchronization signal for device-to-device (D2D) communication by a user equipment (UE) in a wireless communication system is provided. A synchronization signal is received from a base station (BS) to acquire synchronization. Information on capability of the UE related with a synchronization mode is transmitted to the BS. Information on the synchronization mode indicating a first synchronization mode or a second synchronization mode is received from the BS. A reference synchronization of transmitting a D2D signal is determined based on the synchronization mode. When the information on the synchronization mode indicates the second synchronization mode, a synchronization signal is received from a device except for the BS to acquire synchronization for D2D communication.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the National Phase of PCT International ApplicationNo. PCT/KR2015/006144, filed on Jun. 17, 2015, which claims priorityunder 35 U.S.C. 119(e) to U.S. Provisional Application No's. 62/013,517and 62/109,037, filed on Jun. 17, 2014 and Jan. 28, 2015, respectively,all of which are hereby expressly incorporated by reference into thepresent application.

TECHNICAL FIELD

The present invention relates to a wireless communication system, andmore particularly, to a method and device for receiving asynchronization signal for device-to-device (D2D) communication in awireless communication system.

BACKGROUND ART

A structure of a 3GPP LTE (3rd Generation Partnership Project Long TermEvolution; hereinafter, referred as “LTE”) system which is an example ofa wireless communication system to which the present invention may beapplied will be described.

FIG. 1 illustrates a schematic structure a network structure of anevolved universal mobile telecommunication system (E-UMTS). An E-UMTSsystem is an evolved version of the UMTS system and basicstandardization thereof is in progress under the 3rd GenerationPartnership Project (3GPP). The E-UMTS is also referred to as a LongTerm Evolution (LTE) system. For details of the technical specificationsof the UMTS and E-UMTS, refer to Release 7 and Release 8 of “3rdGeneration Partnership Project; Technical Specification Group RadioAccess Network”.

Referring to FIG. 1, the E-UMTS includes a User Equipment (UE), basestations (or eNBs or eNode Bs), and an Access Gateway (AG) which islocated at an end of a network (E-UTRAN) and which is connected to anexternal network. Generally, an eNB can simultaneously transmit multipledata streams for a broadcast service, a multicast service and/or aunicast service.

One or more cells may exist for one BS. The cell provides a downlink oruplink transmission service to several UEs using any one of bandwidthsof 1.25, 2.5, 5, 10, 15 and 20 MHz. Different cells may be set toprovide different bandwidths. A BS controls data transmission orreception to or from a plurality of UEs. The BS transmits downlinkscheduling information to a UE with respect to downlink (DL) data so asto inform the UE of time/frequency domain, coding, data size, HybridAutomatic Repeat and reQuest (HARQ) associated information of data to betransmitted, or the like. The BS transmits uplink scheduling informationto a UE with respect to uplink (UL) data so as to inform the UE oftime/frequency domain, coding, data size, HARQ associated informationused by the UE, or the like. An interface for transmitting user trafficor control traffic can be used between BSs. A Core Network (CN) mayinclude the AG, a network node for user registration of the UE, or thelike. The AG manages mobility of a UE on a Tracking Area (TA) basis. OneTA includes a plurality of cells.

Wireless communication technology has been developed to reach the LTEbased on Wideband Code Division Multiple Access (WCDMA), but demands andexpectations of users and providers have continuously increased. Inaddition, since other aspects of wireless access technology continue toevolve, new advances are required to remain competitive in the future.There is a need for reduction in cost per bit, service availabilityincrease, the use of a flexible frequency band, a simple structure andan open type interface, appropriate power consumption of a UE, etc.

DISCLOSURE Technical Problem

An object of the present invention devised to solve the problem lies ina method and device for receiving a synchronization signal fordevice-to-device (D2D) communication in a wireless communication system.

It is to be understood that both the foregoing general description andthe following detailed description of the present invention areexemplary and explanatory and are intended to provide furtherexplanation of the invention as claimed.

Technical Solution

The object of the present invention can be achieved by providing amethod of receiving a synchronization signal for device-to-device (D2D)communication by user equipment (UE) in a wireless communication system,the method including receiving a synchronization signal from a firsttype device to acquire synchronization, receiving information on asynchronization mode indicating a first synchronization mode or a secondsynchronization mode from the first type device, and when thesynchronization mode is the second synchronization mode, receiving asynchronization signal from the second type device to acquiresynchronization for D2D communication. In another aspect of the presentinvention, provided herein is a user equipment (UE) device for receivinga synchronization signal for device-to-device (D2D) communication in awireless communication system, the UE device including a transceiver fortransmitting and receiving a signal to and from a first type device, asecond type device, or a counterpart UE device of D2D communication, anda processor for processing the signal, wherein the processor receives asynchronization signal from a first type device to acquiresynchronization, receives information on a synchronization modeindicating a first synchronization mode or a second synchronization modefrom the first type device, and receives a synchronization signal fromthe second type device to acquire synchronization for D2D communicationwhen the synchronization mode is the second synchronization mode.

The following features according to the aforementioned embodiments willbe commonly applied.

When the synchronization mode is the first synchronization mode, thesynchronization for D2D communication may be acquired using only thesynchronization signal received from the first type device.

When the synchronization mode is the second synchronization mode, timesynchronization for D2D communication may be acquired based on thesynchronization signal received from the second type device, andfrequency synchronization for D2D communication may be acquired based onthe synchronization signal received from the first type device.

The method may further include receiving information on resourceallocation from the first type device, wherein the information onresource allocation may include information indicating a resource forreceiving a synchronization signal from the second type device.

The first type device may be a base station (BS) that wirelesslycommunicates with the UE, and the second type device may be a deviceexcept for the first type device.

The method may further include transmitting information on supportedcapability of the second synchronization mode to the first type device.

The UE may perform D2D according to synchronization acquired from thesecond type device during a predetermined subframe at a preset timepoint.

Here, when a signal to the first type device and a signal to acounterpart UE of D2D communication are simultaneously scheduled in thepredetermined frame, the signal to the counterpart UE may be prioritizedtop the signal to the first type device.

The method may further include generating a synchronization signalbetween UEs based on a sequence generation root index, and transmittingthe generated synchronization signal between the UEs to a counterpart ofD2D communication, wherein the sequence generation root index is set toa different value in the first synchronization mode and the secondsynchronization mode.

The method may further include transmitting data to a counterpart UE ofD2D communication, wherein a parameter of the data may be differentlyset with respect to the first synchronization mode and the secondsynchronization mode. Here, the parameter of the data may include atleast one of a demodulation reference signal sequence formationparameter and a scrambling sequence formation parameter.

The method may further include receiving a downlink signal from thefirst type device through a specific subframe, and performing D2Dcommunication with an uplink subframe corresponding to the specificsubframe when uplink grant is not included in the downlink signal. Here,the method may further include performing D2D communication in at leastone subframe subsequent to the specific subframe.

It is to be understood that both the foregoing general description andthe following detailed description of the present invention areexemplary and explanatory and are intended to provide furtherexplanation of the invention as claimed.

Advantageous Effects

According to the present invention, a synchronization signal may bereceived in order to effectively perform a device-to-device (D2D)operation in a wireless communication system. In detail, synchronizationfor D2D communication may be effectively acquired.

It will be appreciated by persons skilled in the art that that theeffects that could be achieved with the present invention are notlimited to what has been particularly described hereinabove and otheradvantages of the present invention will be more clearly understood fromthe following detailed description taken in conjunction with theaccompanying drawings.

DESCRIPTION OF DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention, illustrate embodiments of the inventionand together with the description serve to explain the principle of theinvention.

In the drawings:

FIG. 1 is a diagram illustrating a structure of an E-UMTS network as anexample of a wireless communication system;

FIG. 2 is a diagram illustrating a control plane and a user plane of aradio interface protocol between user equipment (UE) and E-UTRAN basedon the 3GPP wireless access network standard;

FIG. 3 is a diagram illustrating physical channels used in a 3GPP LTEsystem and a general method of transmitting a signal using the physicalchannels;

FIG. 4 is a diagram illustrating a structure of a radio frame used in aLTE system;

FIG. 5 is a diagram illustrating resource grid for downlink slot;

FIG. 6 is a diagram illustrating a structure of an uplink subframe;

FIG. 7 is a diagram illustrating a structure of a downlink subframe usedin LTE;

FIG. 8 is a diagram illustrating a communication system applicable tothe present invention;

FIG. 9 is a diagram illustrating an example of an environment to which asynchronization acquiring method according to the present invention isapplicable;

FIG. 10 is a diagram illustrating an example of determining a resourceaccording to the present invention;

FIG. 11 is a diagram illustrating another example of determining aresource according to the present invention;

FIG. 12 is a diagram illustrating a concept of an operation of acquiringsynchronization according to an embodiment of the present invention;

FIG. 13 is a diagram illustrating an uplink scheduling method for a D2Doperation according to the present invention;

FIG. 14 is a diagram illustrating uplink scheduling of an eNB inconsideration of HARQ-ACK transmission according to the presentinvention;

FIG. 15 is a diagram illustrating an example illustrating HARQ-ACKtransmission according to uplink scheduling; and

FIG. 16 is a block diagram of a communication device according to anembodiment of the present invention.

BEST MODE

The following embodiments of the present invention can be applied to avariety of wireless access technologies, for example, CDMA (CodeDivision Multiple Access), FDMA (Frequency Division Multiple Access),TDMA (Time Division Multiple Access), OFDMA (Orthogonal FrequencyDivision Multiple Access), SC-FDMA (Single Carrier Frequency DivisionMultiple Access), and the like. The CDMA may be embodied with wireless(or radio) technology such as UTRA (Universal Terrestrial Radio Access)or CDMA2000. The TDMA may be embodied with wireless (or radio)technology such as GSM (Global System for Mobile communications)/GPRS(General Packet Radio Service)/EDGE (Enhanced Data Rates for GSMEvolution). The OFDMA may be embodied with wireless (or radio)technology such as Institute of Electrical and Electronics Engineers(IEEE) 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802-20, and E-UTRA(Evolved UTRA). The UTRA is a part of the UMTS (Universal MobileTelecommunications System). The 3GPP (3rd Generation PartnershipProject) LTE (long term evolution) is a part of the E-UMTS (EvolvedUMTS), which uses E-UTRA. The 3GPP LTE employs the OFDMA in downlink andemploys the SC-FDMA in uplink. The LTE-Advanced (LTE-A) is an evolvedversion of the 3GPP LTE. WiMAX can be explained by an IEEE 802.16e(WirelessMAN-OFDMA Reference System) and an advanced IEEE 802.16m(WirelessMAN-OFDMA Advanced System).

For clarity, the following description focuses on the 3GPP LTE and 3GPPLTE-A system. However, technical features of the present invention arenot limited thereto.

FIG. 2 is a diagram for structures of control and user planes of radiointerface protocol between a 3GPP radio access network standard-baseduser equipment and E-UTRAN. The control plane means a path on whichcontrol messages used by a user equipment (UE) and a network to manage acall are transmitted. The user plane means a path on which such a datagenerated in an application layer as audio data, internet packet data,and the like are transmitted.

A physical layer, which is a 1st layer, provides higher layers with aninformation transfer service using a physical channel. The physicallayer is connected to a medium access control layer situated above via atransport channel (trans antenna port channel). Data moves between themedium access control layer and the physical layer on the transportchannel. Data moves between a physical layer of a transmitting side anda physical layer of a receiving side on the physical channel. Thephysical channel utilizes time and frequency as radio resources.Specifically, the physical layer is modulated by OFDMA (orthogonalfrequency division multiple access) scheme in DL and the physical layeris modulated by SC-FDMA (single carrier frequency division multipleaccess) scheme in UL.

Medium access control (hereinafter abbreviated MAC) layer of a 2nd layerprovides a service to a radio link control (hereinafter abbreviated RLC)layer, which is a higher layer, on a logical channel. The RLC layer ofthe 2nd layer supports a reliable data transmission. The function of theRLC layer may be implemented by a function block within the MAC. PDCP(packet data convergence protocol) layer of the 2nd layer performs aheader compression function to reduce unnecessary control information,thereby efficiently transmitting such IP packets as IPv4 packets andIPv6 packets in a narrow band of a radio interface.

Radio resource control (hereinafter abbreviated RRC) layer situated inthe lowest location of a 3rd layer is defined on a control plane only.The RRC layer is responsible for control of logical channels, transportchannels and physical channels in association with a configuration, are-configuration and a release of radio bearers (hereinafter abbreviatedRBs). The RB indicates a service provided by the 2nd layer for a datadelivery between the user equipment and the network. To this end, theRRC layer of the user equipment and the RRC layer of the networkexchange a RRC message with each other. In case that there is an RRCconnection (RRC connected) between the user equipment and the RRC layerof the network, the user equipment lies in the state of RRC connected(connected mode). Otherwise, the user equipment lies in the state of RRCidle (idle mode). A non-access stratum (NAS) layer situated at the topof the RRC layer performs such a function as a session management, amobility management and the like.

A single cell consisting of an eNode B (eNB) is set to one of 1.25 MHz,2.5 MHz, 5 MHz, 10 MHz, 15 MHz, and 20 MHz of bandwidths and thenprovides a downlink or uplink transmission service to a plurality ofuser equipments. Different cells can be configured to providecorresponding bandwidths, respectively.

DL transport channels for transmitting data from a network to a userequipment include a BCH (broadcast channel) for transmitting a systeminformation, a PCH (paging channel) for transmitting a paging message, adownlink SCH (shared channel) for transmitting a user traffic or acontrol message and the like. DL multicast/broadcast service traffic ora control message may be transmitted on the DL SCH or a separate DL MCH(multicast channel). Meanwhile, UL transport channels for transmittingdata from a user equipment to a network include a RACH (random accesschannel) for transmitting an initial control message, an uplink SCH(shared channel) for transmitting a user traffic or a control message. Alogical channel, which is situated above a transport channel and mappedto the transport channel, includes a BCCH (broadcast channel), a PCCH(paging control channel), a CCCH (common control channel), a MCCH(multicast control channel), a MTCH (multicast traffic channel) and thelike.

FIG. 3 is a diagram for explaining physical channels used for 3GPPsystem and a general signal transmission method using the physicalchannels.

If a power of a user equipment is turned on or the user equipment entersa new cell, the user equipment may perform an initial cell search jobfor matching synchronization with an eNode B and the like [S301]. Tothis end, the user equipment may receive a primary synchronizationchannel (P-SCH) and a secondary synchronization channel (S-SCH) from theeNode B, may be synchronized with the eNode B and may then obtaininformation such as a cell ID and the like. Subsequently, the userequipment may receive a physical broadcast channel from the eNode B andmay be then able to obtain intra-cell broadcast information. Meanwhile,the user equipment may receive a downlink reference signal (DL RS) inthe initial cell search step and may be then able to check a DL channelstate.

Having completed the initial cell search, the user equipment may receivea physical downlink shared control channel (PDSCH) according to aphysical downlink control channel (PDCCH) and an information carried onthe physical downlink control channel (PDCCH). The user equipment may bethen able to obtain a detailed system information [S302].

Meanwhile, if a user equipment initially accesses an eNode B or does nothave a radio resource for transmitting a signal, the user equipment maybe able to perform a random access procedure to complete the access tothe eNode B [S303 to S306]. To this end, the user equipment may transmita specific sequence as a preamble on a physical random access channel(PRACH) [S303/S305] and may be then able to receive a response messageon PDCCH and the corresponding PDSCH in response to the preamble[S304/S306]. In case of a contention based random access procedure(RACH), it may be able to additionally perform a contention resolutionprocedure.

Having performed the above mentioned procedures, the user equipment maybe able to perform a PDCCH/PDSCH reception [S307] and a PUSCH/PUCCH(physical uplink shared channel/physical uplink control channel)transmission [S308] as a general uplink/downlink signal transmissionprocedure. In particular, the user equipment receives a DCI (downlinkcontrol information) on the PDCCH. In this case, the DCI contains such acontrol information as an information on resource allocation to the userequipment. The format of the DCI varies in accordance with its purpose.Meanwhile, control information transmitted to an eNode B from a userequipment via UL or the control information received by the userequipment from the eNode B includes downlink/uplink ACK/NACK signals,CQI (Channel Quality Indicator), PMI (Precoding Matrix Index), RI (RankIndicator) and the like. In case of 3GPP LTE system, the user equipmentmay be able to transmit the aforementioned control information such asCQI/PMI/RI and the like on PUSCH and/or PUCCH.

The structure of a radio frame of 3GPP LTE system will be described withreference to FIG. 4.

In a cellular Orthogonal Frequency Division Multiplexing (OFDM) radiopacket communication system, uplink/downlink data packets aretransmitted in subframes. One subframe is defined as a predeterminedtime interval including a plurality of OFDM symbols. The 3GPP LTEstandard supports a type 1 radio frame structure applicable to FrequencyDivision Duplex (FDD) and a type 2 radio frame structure applicable toTime Division Duplex (TDD).

FIG. 4(a) illustrates the type-1 radio frame structure. A downlink radioframe is divided into ten subframes. Each subframe includes two slots inthe time domain. The time taken to transmit one subframe is defined as atransmission time interval (TTI). For example, a subframe may have aduration of 1 ms and one slot may have a duration of 0.5 ms. A slot mayinclude a plurality of OFDM symbols in the time domain and includes aplurality of resource blocks (RBs) in the frequency domain. Since 3GPPLTE adopts OFDMA for downlink, an OFDM symbol represents one symbolperiod. An OFDM symbol may be referred to as an SC-FDMA symbol or asymbol period. A resource block (RB), which is a resource allocationunit, may include a plurality of consecutive subcarriers in a slot.

The number of OFDM symbols included in one slot depends on theconfiguration of a cyclic prefix (CP). CPs are divided into an extendedCP and a normal CP. For a normal CP configuring each OFDM symbol, a slotmay include 7 OFDM symbols. For an extended CP configuring each OFDMsymbol, the duration of each OFDM symbol extends and thus the number ofOFDM symbols included in a slot is smaller than in the case of thenormal CP. For the extended CP, a slot may include, for example, 6 OFDMsymbols. When a channel status is unstable as in the case of high speedmovement of a UE, the extended CP may be used to reduce inter-symbolinterference.

When the normal CP is used, each slot includes 7 OFDM symbols, and thuseach subframe includes 14 OFDM symbols. In this case, the first two orthree OFDM symbols of each subframe may be allocated to a physicaldownlink control channel (PDCCH) and the other three OFDM symbols may beallocated to a physical downlink shared channel (PDSCH).

FIG. 4(b) illustrates the type-2 radio frame structure. The type-2 radioframe includes two half frames, each of which has 5 subframes, adownlink pilot time slot (DwPTS), a guard period (GP), and an uplinkpilot time slot (UpPTS). Each subframe includes two slots.

The DwPTS is used for initial cell search, synchronization, or channelestimation in a UE, whereas the UpPTS is used for channel estimation inan eNB and UL transmission synchronization in a UE. The GP is providedto eliminate interference taking place in UL due to multipath delay of aDL signal between DL and UL. Regardless of the type of a radio frame, asubframe of the radio frame includes two slots.

The current 3GPP standard document defines configuration of the specialsubframe as shown in Table 2 below. Table 2 shows DwPTS and UpPTS givenwhen TS=1/(15000*2048), and the other region is configured as a GP.

TABLE 1 Normal cyclic prefix in downlink UpPTS Extended cyclic prefix indownlink Normal Extended UpPTS Special subframe cyclic prefix cyclicprefix Normal cyclic Extended cyclic configuration DwPTS in uplink inuplink DwPTS prefix in uplink prefix in uplink 0  6592 · T_(s) 2192 ·T_(s) 2560 · T_(s)  7680 · T_(s) 2192 · T_(s) 2560 · T_(s) 1 19760 ·T_(s) 20480 · T_(s) 2 21952 · T_(s) 23040 · T_(s) 3 24144 · T_(s) 25600· T_(s) 4 26336 · T_(s)  7680 · T_(s) 4384 · T_(s) 5120 · T_(s) 5  6592· T_(s)  4384 · T_(s) 5120 · T_(s) 20480 · T_(s) 6 19760 · T_(s) 23040 ·T_(s) 7 21952 · T_(s) 12800 · T_(s) 8 24144 · T_(s) — — — 9 13168 ·T_(s) — — —

In the LTE TDD system, uplink/downlink subframe configurations (UL/DLconfigurations) are given as shown in Table 1 below.

TABLE 2 Downlink- to-Uplink Uplink- Switch- downlink point Subframenumber configuration periodicity 0 1 2 3 4 5 6 7 8 9 0 5 ms D S U U U DS U U U 1 5 ms D S U U D D S U U D 2 5 ms D S U D D D S U D D 3 10 ms  DS U U U D D D D D 4 10 ms  D S U U D D D D D D 5 10 ms  D S U D D D D DD D 6 5 ms D S U U U D S U U D

In Table 1, D denotes a downlink subframe, U denotes an uplink subframe,and S denotes the special subframe. Table 1 also showsdownlink-to-uplink switch-point periodicity in uplink/downlink subframeconfiguration of each system.

The structure of the above radio frame is just example. The number of asubframe, the number of slot included in a subframe or the number ofsymbol included in a slot included in the radio frame can be changed.

FIG. 5 is a diagram illustrating a resource grid for a downlink slot.

Referring to FIG. 5, a DL slot includes N_(symb) ^(DL) OFDM symbols inthe time domain and N_(RB) ^(DL) resource blocks in the frequencydomain. Since each resource block includes N_(sc) ^(RB) subcarriers, theDL slot includes N_(RB) ^(DL)×N_(sc) ^(RB) subcarriers in the frequencydomain. Although FIG. 5 illustrates that the DL slot includes seven OFDMsymbols and the resource block includes twelve subcarriers, it is to beunderstood that the downlink slot and the resource block are not limitedthereto. For example, the number of OFDM symbols included in thedownlink slot may be varied depending on a length of CP (cyclic prefix).

Each element on the resource grid is referred to as a resource element(RE). One resource element is indicated by one OFDM symbol index and onesubcarrier index. One RB includes N_(symb) ^(DL)×N_(sc) ^(RB) number ofREs. The number N_(RB) ^(DL) of RBs included in the DL slot depends on aDL transmission bandwidth configured in a cell.

FIG. 6 illustrates a structure of an uplink subframe applicable toembodiments of the present invention.

Referring to FIG. 6, a UL subframe may be divided into a control regionand a data region in the frequency domain. A PUCCH for carrying uplinkcontrol information is allocated to the control region and a PUSCH forcarrying user data is allocated to the data region. In the LTE system, aUE does not simultaneously transmit the PUCCH and the PUSCH to maintaina single carrier property. However, in the LTE-A system, a PUCCH signaland a PUSCH signal can be simultaneously transmitted due to theintroduction of carrier aggregation technology. The PUCCH for one UE isallocated to an RB pair in a subframe. RBs belonging to the RB pairoccupy different subcarriers in respective two slots. This is calledthat the RB pair allocated to the PUCCH is frequency-hopped in a slotboundary.

FIG. 7 is a diagram illustrating a structure of a downlink subframeapplicable to embodiments of the present invention.

Referring to FIG. 7, a maximum of three OFDM symbols from OFDM symbolindex #0 of a first slot in a subframe correspond to a control region tobe assigned with a control channel. The remaining OFDM symbolscorrespond to a data region to be assigned with a PDSCH. Examples ofdownlink control channels used in the 3GPP LTE system includes aphysical control format indicator channel (PCFICH), a physical downlinkcontrol channel (PDCCH), a physical hybrid ARQ indicator channel(PHICH), etc.

The Physical Control Format Indicator Channel (PCFICH) informs the UE ofthe number of OFDM symbols used for the PDCCH per subframe. The PCFICHis located at a first OFDM symbol and is configured prior to the PHICHand the PDCCH. The PCFICH includes four Resource Element Groups (REGs)and the REGs are dispersed in the control region based on a cellidentity (ID). One REG includes four resource elements (REs). The REindicates minimum physical resource defined as one subcarrier x one OFDMsymbol. The PCFICH has a value of 1 to 3 or 2 to 4 according tobandwidth and is modulated using a Quadrature Phase Shift Keying (QPSK)scheme.

The Physical Hybrid-ARQ Indicator Channel (PHICH) is used to carry HARQACK/NACK for uplink transmission. That is, the PHICH refers to a channelvia which DL ACK/NACK information for uplink HARQ is transmitted. ThePHICH includes one REG and is scrambled on a cell-specific basis.ACK/NACK is indicated by one bit and is modulated using a binary phaseshift keying (BPSK) scheme. The modulated ACK/NACK is repeatedly spreadwith a spreading factor (SF) of 2 or 4. A plurality of PHICHs mapped tothe same resources configures a PHICH group. The number of PHICHsmultiplexed in the PHICH group is determined according to the number ofspreading codes. The PHICH (group) is repeated three times in order toobtain diversity gain in a frequency region and/or time region.

The Physical Downlink Control Channel (PDCCH) is allocated to the firstn OFDM symbols of a subframe. Here, n is an integer of 1 or more and isindicated by a PCFICH. The PDCCH includes one or more Control ChannelElements (CCEs). The PDCCH informs each UE or a UE group of informationassociated with resource allocation of a Paging Channel (PCH) and aDownlink-Shared Channel (DL-SCH), both of which are transport channels,uplink scheduling grant, HARQ information, etc. The paging channel (PCH)and the downlink-shared channel (DL-SCH) are transmitted through aPDSCH. Accordingly, the eNB and the UE transmit and receive data throughthe PDSCH except for specific control information or specific servicedata.

Information indicating to which UE (one or a plurality of UEs) data ofthe PDSCH is transmitted and information indicating how the UEs receiveand decode the PDSCH data are transmitted in a state of being includedin the PDCCH. For example, it is assumed that a specific PDCCH isCRC-masked with a Radio Network Temporary Identity (RNTI) “A”, andinformation about data transmitted using radio resource (e.g., frequencylocation) “B” and transmission format information (e.g., transmissionblock size, modulation scheme, coding information, or the like) “C” istransmitted via a specific subframe. In this case, one or more UEslocated within a cell monitor a PDCCH using its own RNTI information,and if one or more UEs having “A” RNTI are present, the UEs receive thePDCCH and receive the PDSCH indicated by “B” and “C” through theinformation about the received PDCCH.

Synchronization Signal

Hereinafter, a synchronization signal will be described.

When a UE is powered on or newly attempts to access a cell, the UE mayobtain time and frequency synchronization with the cell and perform acell search procedure of detecting a physical layer cell identity(NcellID) of the cell. To this end, the UE may receive asynchronization, for example, a primary synchronization signal (PSS) anda secondary synchronization signal (SSS) from an eNB and synchronizes toits timing to the eNB to obtain information on a cell identity, and soon.

In detail, the PSS may be used as PSSd(n) by defining a Zadoff-Chu (ZC)sequence with a length 63 in the frequency domain according to Equation1 below in order to acquire time domain synchronization and/or frequencydomain synchronization such as OFDM symbol synchronization and slotsynchronization.

$\begin{matrix}{{d_{u}(n)} = \left\{ \begin{matrix}e^{{- j}\frac{\pi\;{{un}{({n + 1})}}}{63}} & {{n = 0},1,\ldots\mspace{14mu},30} \\e^{{- j}\frac{\pi\;{u{({n + 1})}}{({n + 2})}}{63}} & {{n = 31},32,\ldots\mspace{14mu},61}\end{matrix} \right.} & \left\lbrack {{Equation}\mspace{14mu} 1} \right\rbrack\end{matrix}$

In Equation 1 above, u is a ZC root sequence index and is defined in acurrent LTE system as shown in Table 3 below.

TABLE 3 N_(ID) ⁽²⁾ Root index u 0 25 1 29 2 34

SSS may be used to obtain frame synchronization, a cell group ID, and/orcell CP configuration (i.e., usage information of normal CP or extensionCP) and may be configured via interleaving combination of two binarysequences with a length 31. That is, the SSS sequence is d(0), . . .,d(61) and has a total length of 62. The SSS sequence may be differentlydefined according to whether the sequence is transmitted in subframe #0or #5 according to Equation 2 below. In Equation 2, n is an integerbetween 0 and 30.

$\begin{matrix}{{d\left( {2\; n} \right)} = \left\{ {{\begin{matrix}{{s_{0}^{(m_{0})}(n)}{c_{0}(n)}} & {{in}\mspace{14mu}{subframe}\mspace{14mu} 0} \\{{s_{1}^{(m_{1})}(n)}{c_{0}(n)}} & {{in}\mspace{14mu}{subframe}\mspace{14mu} 5}\end{matrix}{d\left( {{2\; n} + 1} \right)}} = \left\{ \begin{matrix}{{s_{1}^{(m_{1})}(n)}{c_{1}(n)}{z_{1}^{(m_{0})}(n)}} & {{in}\mspace{14mu}{subframe}\mspace{14mu} 0} \\{{s_{0}^{(m_{0})}(n)}{c_{1}(n)}{z_{1}^{(m_{1})}(n)}} & {{in}\mspace{14mu}{subframe}\mspace{14mu} 5}\end{matrix} \right.} \right.} & \left\lbrack {{Equation}\mspace{14mu} 2} \right\rbrack\end{matrix}$

In more detail, a synchronization signal may be transmitted in a firstslot of subframe #0 and a first slot of subframe #5 in consideration of4.6 ms that is a length of a global system for mobile communication(GSM) frame in order to easily measure inter radio access technology(RAT). In particular, the PSS may be transmitted in a last OFDM symbolof a first slot of subframe #0 and a last OFDM symbol of a first slot ofsubframe #5 and the SSS may be transmitted in the last OFDM symbol butone of a first slot of subframe #0 and the last OFDM symbol but one of afirst slot of subframe #5. A boundary of a corresponding wireless framemay be detected through the SSS. The PSS may be transmitted in the lastOFDM symbol of a corresponding slot and the SSS may be transmitted in anOFDM symbol immediately before the PSS.

SS may indicate a total of 504 unique physical layer cell IDs via acombination of three PSSs and 168 SSs. In other words, the physicallayer cell IDs may be grouped into 168 physical-layer cell-identitygroups, each of which includes three unique identities, such that eachphysical layer cell ID is a portion of only one physical-layercell-identity group. Accordingly, the physical layer cell identifyNcellID may be uniquely defined by number N(1)ID in a range of 0 to 167indicating a physical-layer cell-identity and number N(2)ID of 0 to 2indicating the physical-layer identify in the physical-layercell-identity. A UE may detect the PSS to recognize one of three uniquephysical-layer identifiers and detect the SSS to identify one of 168physical layer cell IDs associated with the physical-layer identity.

The PSS may be transmitted every 5 ms and, thus, the UE may detect thePSS to recognize a corresponding subframe as one of subframe #0 andsubframe #5 but may not know in detail whether the correspondingsubframe is subframe #0 or subframe #5. Accordingly, the UE may notrecognize a boundary of a radio frame using only the PSS. That is, framesynchronization may not be acquired using only the PSS. The UE maydetect the SSS that is transmitted twice in one radio frame andtransmitted as different sequences and detect a boundary of a radioframe.

As such, for cell search/re-search, the UE may receive the PSS and theSSS from the eNB and synchronize its timing to the eNB to acquireinformation on a cell identify (ID) and so on. Then, the UE may receivebroadcast information in a cell managed by the eNB on a PBCH.

Device to Device (D2D) Communication

When D2D communication is applied to the aforementioned wirelesscommunication system (e.g., 3GPP LTE system or 3GPP LTE-A system), adetailed method for performing D2D communication will be describedbelow.

Hereinafter, a communication environment between devices according tothe present invention will be described briefly.

D2D communication literally refers to communication between electronicdevices. In a broad sense, D2D communication refers to wired or wirelesscommunication between electronic devices or communication between amachine and a device controlled by the human. Recently, in general, D2Dcommunication refers to wireless communication between electronicdevices, which is performed irrespective of the human.

FIG. 8 is a diagram for explanation of a concept of D2D communication.FIG. 8 illustrates D2D or UE-to-UE communication method as an example ofD2D communication and data exchange between UEs may be performed withoutpassing through an eNB. As such, link that is directly establishedbetween devices may be referred to as a D2D link or a side link. D2Dcommunication advantageously has low latency and requires a small amountof radio resources compared with a conventional eNB-centeredcommunication method. Here, the UE refers to a user terminal but may beconsidered as a kind of UE when network equipment such as an eNBtransmits and receives a signal according to a communication methodbetween UEs.

In order to perform D2D communication, two UEs need to acquire time andfrequency synchronization therebetween. In general, when two UEs arewithin coverage of the eNB, two UEs may be synchronized with PSS/SSS,CRS, or the like transmitted from an eNB and time/frequencysynchronization may be maintained in a level in which signals arecapable of being transmitted and received directly between the two UEs.Here, a synchronization signal for D2D communication is referred to as aD2D synchronization signal (D2DSS). A UE that is synchronized with aspecific eNB may transmit a D2DSS based on eNB synchronization. TheD2DSS may be a D2D signal transmitted in order to form synchronizationwith another UE by a UE and another UE may detect the D2DSS so as toacquire synchronization with the corresponding UE. In particular, in thecase of a UE present outside coverage of an eNB to which thecorresponding UE synchronizes its timing (that is, in the case of a UEthat is connected to another eNB or is not connected to any eNB),synchronization may be acquired through this operation. The D2DSS mayinclude a signal such as PSS/SSS of an LTE system. As such, PSS/SSS (ormodified signal of PSS/SSS) transmitted for D2D communication isreferred to as a primary D2D synchronization signal (PD2DSS) and asecondary D2D synchronization signal (SD2DSS). The PD2DSS may be used toacquire schematic timing and based on a ZC sequence like PSS of an LTEsystem. In addition, the SD2DSS may be used for more accuratesynchronization and based on an m-sequence like SSS of an LTE system. Aphysical D2D synchronization channel (PD2DSCH) may refer to a physicalchannel for carrying information required for synchronization, such as asystem bandwidth, a radio frame, and a subframe index.

A UE outside coverage of an eNB may also transmit D2DSS and may allowanother UE to synchronize its timing to the UE. The UE may pre-set atype of D2DSS used according to a state of coverage of a transmission UEof D2DSS in order to allow a UE that detects D2DSS to differentiatewhether the corresponding D2DSS is caused from a UE inside coverage ofthe eNB or a UE outside the coverage of the eNB.

The aforementioned D2D synchronizing method may give priority tosynchronization provided by a network. In more detail, a UE may mostpreferentially select a synchronization signal transmitted directly bythe eNB in order to determine transmission synchronization of the UE andwhen the UE is outside coverage of the eNB, D2DSS transmitted by a UEwithin coverage of the eNB is preferentially synchronized. According tothis principle, a UE may synchronize timing provided by a network ifpossible and a D2D operation may be smoothly multiplexed with anexisting network operation (a transmission and reception operationbetween an eNB and a UE). For example, one subframe performs an existingnetwork operation and a next subframe performs D2D.

On the other hand, the aforementioned conventional D2D synchronizationmethod is determined to preferentially use network timing and, thus, aUE needs to frequently change synchronization according to a networksituation and there is the possibility that synchronization acquisitionfails during this procedure. In detail, a UE determines timing of D2Dsignal transmission based on synchronization with a serving cell and,thus, when the UE may be moved and a serving cell is changed, timesynchronization may also be changed and a transmission UE may converttiming and predetermined latency for re-recognizing the converted timingby a reception UE during this procedure. In particular, transmission andreception UEs lose synchronization therebetween during the latency and,thus, temporary D2D transmission and reception may not be possible.

When adjacent cells are not time-synchronized, a UE of one cell needs tomaintain a plurality of synchronizations while separately trackingtiming of each cell in order to receive a D2D signal of a UE of anothercell and D2DSS transmission for synchronization for each cell may have arelatively long period due to a problem due to the overhead. Forexample, D2DSS transmission may have a period of minimum 40 ms, whichcorrespond to a very long period compared with 5 ms at which an eNBtransmits PSS/SSS. Accordingly, as a D2DSS transmission period isincreased, the possibility that synchronization performance using D2DSSis reduced is high and latency of time for D2D synchronization time maybe caused.

Hereinafter, according to the present invention, a method of morerapidly forming D2D synchronization when D2D UEs already have anexternal synchronization source separated from a source provided by anetwork by employing an external synchronization source in order toovercome a problem in terms of a D2D synchronization procedure will bedescribed. For convenience of description, a device for transmitting anexisting synchronization signal to a UE, for example, a device fortransmitting a synchronization signal to a UE or synchronized with theUE according to an existing synchronization procedure or a device fortransmitting a D2D synchronization signal to a corresponding UE orsynchronized with the UE will be referred to as a first type device andan external synchronization source will be referred to as a second typedevice. A conventional D2D synchronization method is referred to as afirst synchronization mode and a new D2D synchronization method using asecond type device is referred to as a second synchronization mode.

Here, the first type device may be an eNB (or network), a D2D terminal,or the like and a representative second type device may be a globalpositioning system (GPS). For example, it is assumed that a UE using thesecond synchronization mode includes a GPS receiver installed thereinand acquires time synchronization based on a signal provided by a GPSsatellite. However, the second type device is not limited to a devicesuch as GPS. As necessary, an eNB other than the first type device maycorrespond to the second type device as necessary.

FIG. 9 is a diagram illustrating an example of an environment to which asynchronization acquiring method according to the present invention isapplicable.

In particular, as illustrated in FIG. 9, a UE installed in a vehicle mayalways drive a GPS receiver without worry about battery consumption andthe possibility that the UE already drives a GPS receiver for a purposeother than D2D communication, for example, a purpose of a navigationdevice in the case of a vehicle is high and, thus, the UE may beappropriate to use the second synchronization mode. In the case of ashadow area such as a tunnel in which a GPS signal is not capable ofbeing received from a satellite, equipment for relaying a GPS signal maybe installed so as to continuously provide the second type device.

Hereinafter, detailed methods of executing the proposed secondsynchronization mode will be described.

In order to execute the second synchronization mode, a second typedevice may be required. In this regard, some UEs may not originallyexecute this function for the reason of complexity for embodying a UE,battery consumption, or the like or even if this function is embodied,the function may not be temporally used. Accordingly, in order toexecute the second synchronization mode, an eNB needs to recognizewhether a separate UE has capability for executing the secondsynchronization mode. The UE may transmit information on whether thecorresponding UE has capability for executing the second synchronizationmode and/or whether the UE currently executes the first and secondsynchronization modes as information on capability to the eNB.

The eNB may perform an appropriate operation based on the information.For example, the eNB may adjust the amount of resources forcorresponding synchronization-based D2D, based on the number of UEs thatexecute the second synchronization mode. In addition, for example, theeNB may recognize that a UE that executes the second synchronizationmode is capable of performing D2D with a different subframe boundaryfrom network timing by a first type device and reflect the recognitionresult to uplink scheduling, as described later. In general, the secondsynchronization mode is more complex than the first synchronization modeand, thus, a UE that is capable of executing the second synchronizationmode may be defined to execute the first synchronization mode.

The first type device (e.g., eNB) may notify the UE of the secondsynchronization mode and whether D2D signal transmission is permittedaccording to the second synchronization mode within coverage the firsttype device. In this case, whether D2D signal transmission is permittedaccording to the second synchronization mode, to all UEs. Here, thefirst type device may transmit information on a synchronization modeindicating the first synchronization mode or the second synchronizationmode to a UE. For example, when many UEs that transmit to a specificcell a D2D signal according to a second synchronization mode aredistributed, if frequent transmission of corresponding UEs is notmatched with cell synchronization and collision with uplink transmissionis determined to occur with the high possibility, an eNB may indicatethat a D2D signal is transmitted in a corresponding cell according tothe first synchronization mode. That is, D2D signal transmissionaccording to the second synchronization mode may be indicated to bestopped or prevented.

When D2D signal transmission according to the second synchronizationmode is permitted, resources to be used in a second synchronization modeand D2D signal transmission according thereto may also be broadcast. Inthis case, the first type device may transmit resource allocationinformation including information indicating resources for receiving asynchronization signal from the second type device. Similarly, the firsttype device may transmit information indicating a resource fortransmitting a D2D signal according to the second synchronization modeto a UE. However, as described later, a resource for transmitting a D2Dsignal according to the second synchronization mode may be preset basedon a specific time point. Here, as described later, when the secondsynchronization mode, a boundary of the subframe (hereinafter, thesubframe boundary by the second type device) may not be matched with asubframe boundary at network timing and, thus, only a frequency resourcemay be determined, Needless to say, when a network synchronizes itstiming with a second type device, a time resource may also bedetermined.

FIG. 10 is a diagram illustrating an example of determining a secondsynchronization mode and/or D2D signal transmission according to theretoin a specific ell. Here, it is assumed that a network does not have asecond type device and, thus, a subframe boundary defined by the secondsynchronization mode is not matched with a boundary defined by an eNB.Referring to FIG. 10, the specific ell may set a resource to be used inthe second synchronization mode and/or D2D signal transmission accordingthereto by a UE in an uplink band of the specific cell.

FIG. 11 also illustrates an example of determining a resource to be usedin a second synchronization mode and/or D2D signal transmission in aspecific cell and indicates resource allocation to FIG. 10. Here,specifically, a resource to be used in the second synchronization modeand/or D2D signal transmission according to thereto are set outside anuplink band. In particular, this setting may be useful when the secondsynchronization mode is not matched with uplink and thus it is difficultto perform multiplexing together. In this case, in terms of a UE thatattempts to operate in the second synchronization mode, a resource of asecond synchronization mode is indicated outside a bandwidth of anuplink cell associated with a cell for determining a resource of thesecond synchronization mode through a specific downlink link cell, inparticular, downlink. Although not illustrated in FIG. 11, resources tobe used in the second synchronization mode and/or D2D signaltransmission according to thereto are positioned outside both oppositesides of a bandwidth of an uplink cell and constitute two frequencysubregions that are spaced apart from each other. Here, a D2D channelmay be transmitted by hopping the two frequency subregions so as toacquire frequency diversity.

Hereinafter, a method of acquiring synchronization when a D2D UE uses asecond synchronization mode will be described.

First, in the case of time synchronization, all UEs may be deemed to besynchronized with each other based on the aforementioned second typedevice. Accordingly, a D2D subframe boundary when a secondsynchronization mode is used in a predetermined time unit based on aspecific time point in a time defined by the second type device may bedetermined. Here, the specific time point as reference may be pre-inputto UEs. Hereinafter, the specific time point will be referred to as anexternal synchronization reference. In addition, the predetermined timeunit may also be pre-set and, for example, the D2D subframe boundary maybe set to be determined in a unit of 1 ms. Accordingly, a UE that usesthe second synchronization mode may pre-acquire time synchronizationincluding one subframe boundary connected to the second type device.Accordingly, the UE may recognize start and end points as a specific D2Dsubframe without a separate synchronization procedure and transmit orreceive a D2D signal according to the recognition result.

When different D2D signals are transmitted according to time, when andwhich signal is transmitted may be predetermined. For example, when asubframe is determined using the aforementioned method, where D2DSS istransmitted may also be determined.

In order to acquire frequency synchronization of a D2D signal, thefollowing method may be used. As one method, similarly to signalsynchronization, a method of generating a reference signal of frequencysynchronization using a second type device may be used. However, in thiscase, it may be general to require a separate device. In particular, aradio frequency of radio signal transmission and reception is variouslyindicated according to network configuration and, thus, a device forguiding various frequency synchronization reference signals from thesecond type device may have relatively high cost. On the other hand,frequency synchronization does not conclusively affect an operationcompared with time synchronization despite slight error and simply andslightly degrade performance and, thus, it may not be necessary to guidefrequency synchronization from the second type device. In this case, asa second method, time synchronization may be guided from the second typedevice and frequency synchronization may be guide from a network, asdescribed above. In particular, the UE needs to acquire frequencysynchronization of a network in order to transmit a signal to an eNBand, thus, when the frequency synchronization is re-used,synchronization may be completed without separation complex embodiment.In addition, dynamic conversion is not advantageously necessary as amethod of synchronizing a frequency of a transmission signal with afrequency of the eNB at a specific time at an uplink band during uplinktransmission and guiding a frequency of a transmission signal from asecond type device at a different time point. Hereinafter, a method ofacquiring frequency synchronization for D2D transmission will bedescribed in more detail.

FIG. 12 is a diagram illustrating a concept of an operation of acquiringtime synchronization from a second type device and acquiring frequencysynchronization from a network according to an embodiment of the presentinvention.

As illustrated in FIG. 12, when frequency synchronization is guided froma network in the second synchronization mode, it may be still necessaryto transmit D2DSS for synchronization with a UE outside coverage of anetwork. However, the UE outside coverage of a network also recognizestime synchronization using the second type device and, thus, the UE maydirectly attempt D2DSS in a subframe defined according thereto. As aresult, a procedure of acquiring time synchronization using D2DSS may beomitted and time latency due to detection error may be remarkablyreduced.

With regard to the feature of D2DSS used in the second synchronizationmode, it may not be necessary to acquire time synchronization throughthe D2DSS and, thus, the frequency position may be more flexible than inthe first synchronization mode. Only a center frequency region of anuplink bandwidth is determined to be used by D2DSS in the firstsynchronization mode and, thus, when a UE first acquires timesynchronization, a position in a frequency resource may be fixed toeasily acquire synchronization but this limit may not be necessary inthe second synchronization mode. That is, D2DSS of the secondsynchronization mode may be permitted to be transmitted even if acurrent region is not a center region of an uplink bandwidth.

However, in order to exclude excessively high possibility, candidates offrequency resources for transmitting D2DSS may be limited to apredetermined number. Alternatively, in the case of the secondsynchronization mode, even UEs positioned in a different cell maintainthe same synchronization and, thus, all UEs in a network transmit andreceive the same D2DSS with a transmission resource and transmissionsequence of a single D2DSS. However, when all UEs simultaneouslytransmit the D2DSS, there is no change to receive the D2DSS and, thus,each UE may stochastically transmit every D2DSS transmission time point.In this case, in order to prevent only D2DSS from being continuouslytransmitted or received, when the D2DSS is transmitted once, thepossibility of next transmission may be reduced. Alternatively, when theD2DSS is not transmitted once, the possibility of next transmission maybe increased.

D2DSS of the second synchronization mode with this attribute needs to bedifferentiated from D2DSS of the first synchronization mode and, thus,different sequences may be used in the first synchronization mode andthe second synchronization mode. That is, a sequence that is not used inthe D2DSS of the first synchronization mode may be used. For example, anumber other than a sequence generation root index that has been used togenerate the first synchronization mode D2DSS may be used as a rootindex for generation of the second synchronization mode D2DSS.

Accordingly, various parameters of a D2D data channel based on thesecond synchronization mode may be differently set from the case basedon the first synchronization mode so as to differentiate two cases. Forexample, a DMRS sequence formation parameter and/or a D2D data channelscrambling sequence formation parameter and/or a CRC mask for a D2D datachannel may be differently set.

Hereinafter, a D2D transmission scheduling method when the secondsynchronization mode is used will be described.

First, it is difficult to simultaneously transmit data to D2D at thesame time by one UE because an uplink transmission signal of LTEbasically is determined to occupy consecutive RBs. Accordingly, when D2Dsignal transmission and signal transmission to an eNB collide with eachother at the same time, only one of these needs to be transmitted.Existing D2D is designed to apply priority to signal transmission to aneNB and to stop D2D signal transmission. However, in the case of D2Dusing the aforementioned second synchronization mode, a very emergentsignal needs to be rapidly transmitted for D2D and, thus, in the case ofat last one some of emergent D2D signals, signal transmission to an eNBmay be stopped and D2D may be transmitted.

In detail, when a signal to the first type device and a signal to acounterpart terminal of D2D are simultaneously scheduled in a period inwhich the second synchronization is applied, a signal to the counterpartterminal may have priority to a signal (signal between eNB and UE) tothe first type device.

This differentiation may be performed in a subframe unit defined by, inparticularly, the second type device. For example, D2D transmission in aspecific subframe set has priority to signal transmission to an eNB butD2D transmission in the other subframe sets may have priority to signaltransmission to an eNB.

As illustrated in FIGS. 10 and 11, a subframe used by an eNB and asubframe used in the second synchronization mode may have boundariesthat are not matched with each other. Accordingly, it is difficult toalternately perform signal transmission to an eNB and D2D signaltransmission and reception in a subframe unit.

As one method for overcoming this issue, during scheduling oftransmitting a signal to an eNB, the eNB may continuously empty at leasttwo subframes, that is, the eNB may perform scheduling such that thecorresponding UE may not transmit any uplink signal in the two subframesto indicate one complete D2D subframe irrespective of a subframeboundary in at least one subframe. The UE may use the consecutive emptysubframes to receive a D2D signal of another UE or transmit a D2D signalof the UE as necessary.

For example, when uplink grant is not included in a downlink signaltransmitted in a specific subframe, the UE may perform communicationbetween UEs in an uplink subframe corresponding to the specific subframeand perform communication between UEs in at least one subframesubsequent to the specific subframe. For example, when PUSCHtransmission of UL grant received in DL subframe n to an eNB isperformed in UL subframe n+4, if the UE is not capable of receiving anyUL grant in DL subframe n, the UE may recognize that any signal is nottransmitted to an eNB in UL subframe n+4 and transmit a D2D signal usingD2D subframe k started at a middle portion of UL subframe n+4 at anetwork timing. However, in this case, it may be assumed that the eNBmay not transmit UL grant in DL subframe n+1 and may also empty ULsubframe n+5.

FIG. 13 is a diagram illustrating an uplink scheduling method for a D2Doperation according to the present invention.

In order to more smoothly transmit a D2D signal based on the assumptionby a UE, an eNB may notify the UE of information on a subframe in whichUL transmission is not scheduled. Then, the UE may preferentially use asubframe that is not known to be scheduled to transmit a D2D signal.

In the case of the aforementioned operation, an eNB that schedulesuplink of WAN communication between an eNB and a UE may not know asubframe of a UE in which WAN transmission and D2D transmission overlapeach other. When WAN transmission and D2D transmission overlap eachother, even if two signals use different frequency resources, there maybe no limit in that only one of two signals is transmitted according tocapability of a UE. For example, a UE that has no function oftransmitting a signal using inconsecutive subcarriers may transmit onlyone of the two signals. In this case, when D2D transmission isprioritized, WAN transmission may be dropped to degrade WAN performance.In particular, when the dropped WAN transmission is a HARQ-ACK signal ofPDSCH, HARQ-ACK is not capable of being simply transmitted and, thus,there is a problem in that PDSCH is unnecessarily re-transmitted towaste resources.

Hereinafter, in order to overcome this problem, uplink scheduling of aneNB in the case of HARQ-ACK transmission will be described.

To overcome this problem, HARQ-ACK of PDSCH transmitted in one subframemay be designed to be transmitted in at least one subframe of aplurality of UL subframes.

In the case of an existing LTE FDD, HARQ-ACK of PDSCH received insubframe #n may be transmitted in subframe #n+4. In this situation, whenD2D transmission collides with HARQ-ACK transmission in a portion ofsubframe #n+4, it is difficult that the eNB knows whether PDSCH insubframe #n is successfully received. In this case, the HARQ timelinemay be adjusted and, for example, HARQ-ACK of PDSCH received in subframe#n may be determined to be transmitted subframe #n+4 and subframe #n+6.

Specifically, a plurality of subframes for transmitting HARQ-ACK ofPDSCH in one subframe are inconsecutive in order to prevent simultaneouscollision between D2D transmission in one subframe with a mismatchedwith a subframe boundary and HARQ-ACK in two consecutive subframes.Accordingly, even if HARQ-ACK transmission in subframe #n+4 is droppeddue to collision with D2D transmission, when it is possible to transmitHARQ-ACK in subframe #n+6, the problem in terms of waste in PDSCHresources may be overcome. D2D resource may be appropriately configuredto prevent HARQ-ACK transmission from being entirely dropped due tocollision with D2D transmission. In detail, a subframe corresponding toa predetermined time period in an external synchronization reference maybe configured as a D2D transmission subframe and a subframecorresponding to a specific subsequent time period may not be configuredas a D2D transmission subframe. For example, when a subframecorresponding to 1 ms in an external synchronization reference may beconfigured as a D2D transmission subframe and at least subsequent 3 msis not configured as a D2D transmission subframe from the same UE, bothsubframe #n+4 and subframe #n+6 may not be dropped due to collisionbetween HARQ-ACK transmission and D2D transmission in the aforementionedexample.

FIG. 14 is a diagram illustrating uplink scheduling of an eNB inconsideration of HARQ-ACK transmission according to the presentinvention.

Here, it is assumed that external reference subframe #k and subframe#k+4 are used to transmit D2D of a single UE and a gap of 3 ms ispresent between the subframes. Referring to FIG. 14, Case 1, 2, 3, and 4correspond to the case in which a rear portion of subframe #k+4 collidewith a front portion of subframe #n+6, the case in which a front portionof subframe #k+4 collides with a rear portion of subframe #n+6, the casein which a rear portion of subframe #k collides with the front portionof subframe #n+4, and the case in which a front portion of subframe #kcollides with a rear portion of subframe #n+4. Referring to FIG. 14, inany case, HARQ-ACK transmission in both subframe #n+4 and subframe #n+6may not collide. Needless to say, some time periods may be configured asa gap without transmission in the D2D subframe or the WAN subframe.

When HARQ-ACK of one PDSCH is transmitted in a plurality of subframes, atransmitting operation of a UE will be described below in more detail.

First, when HARQ-ACK transmission is not dropped in a subframe (subframe#n+4 of FIG. 14) for transmitting HARQ-ACK, the possibility thatHARQ-ACK will be dropped needs to be considered and, thus, HARQ-ACKneeds to be transmitted. However, when HARQ-ACK is first transmitted andthen it is possible to re-transmit HARQ-ACK in a subframe (subframe #n+6of FIG. 14), one of the following two methods may be used.

Previous transmission of HARQ-ACK may be configured to be sufficient andcorresponding HARQ-ACK in a corresponding subframe may not betransmitted. Accordingly, a WAN transmissions bit number in thecorresponding subframe may be reduced and, thus, transmission power ofanother WAN signal may be reduced or coverage may be increased.

Error may occur in previous transmission of HARQ-ACK and, thus,transmission may be performed as long as a signal is not still droppedin subframe #n+6. The eNB may combine HARQ-ACK over a plurality ofsubframes and attempt to receive HARQ-ACK with relatively highreliability.

When this principle is applied, HARQ-ACK corresponding to PDSCH withrespect to a plurality of subframes in one subframe needs to befrequently transmitted together. FIG. 15 is a diagram illustrating anexample illustrating HARQ-ACK transmission according to uplinkscheduling. Referring to FIG. 15, in subframe #n+6, HARQ-ACK in subframe#n and subframe #n+2 needs to be transmitted together.

The aforementioned present invention may be generalized and applied asthe case in which the second type device is not a cell that ispositioned in a carrier for transmitting a D2D signal. For example, whentiming is acquired from a cell positioned in another carrier but not thecarrier for transmitting a D2D signal and the D2D signal is transmitted,asynchronization may occur between a D2D transmission subframe and a WANtransmission subframe and, in this case, the present invention may beapplied to overcome the same problem. The cell position in anothercarrier but not the carrier for transmitting the D2D signal may be, forexample, an adjacent cell except for a serving cell. The adjacent cellmay be useful in the case in which it is difficult to acquiresynchronization from a source GPS when a UE is positioned in a tunnel orthe like.

FIG. 16 is a block diagram of a communication device 1600 according toan embodiment of the present invention.

Referring to FIG. 16, the communication device 1600 may include aprocessor 1610, a memory 1620, a radio frequency (RF) module 1630, adisplay module 1640, and a user interface module 1650.

The communication device 1600 may be illustrated for convenience ofdescription and, thus, some modules may be omitted. The communicationdevice 1600 may further include required modules. In addition, somemodules of the communication device 1600 may be divided into moredetailed modules. The processor 1610 may be configured to perform theoperation according to the embodiment of the present invention describedwith reference to the drawings. In detail, a detailed operation of theprocessor 1610 will be understood with reference to FIGS. 1 to 34.

The memory 1620 may be connected to the processor 1610 and may store anoperating system, an application, a program code, data, and so on. TheRF module 1630 may be connected to the processor 1610 and convert abaseband signal into a radio signal or convert a radio signal into abaseband signal. To this end, the RF module 1630 may perform analogconversion, amplification, filtering, and frequency upconversion orperform an opposite procedure thereto. The display module 1640 may beconnected to the processor 1610 and may display various informationitems. The display module 1640 is not limited thereto and may use a wellknown element such as a liquid crystal display (LCD), a light emittingdiode (LED), and an organic light emitting diode (OLED). The userinterface module 1650 may be connected to the processor 1610 and may beconfigured with a combination with a well known user interface such as akey pad and a touchscreen.

The above-described embodiments are combinations of elements andfeatures of the present invention in a predetermined manner. Each of theelements or features may be considered selective unless mentionedotherwise. Each element or feature may be practiced without beingcombined with other elements or features. Further, an embodiment of thepresent invention may be constructed by combining parts of the elementsand/or features. Operation orders described in embodiments of thepresent invention may be rearranged. Some constructions of any oneembodiment may be included in another embodiment and may be replacedwith corresponding constructions of another embodiment. In the appendedclaims, claims that are not explicitly dependent on each other may ofcourse be combined to provide an embodiment or new claims can be addedthrough amendment after the application is filed.

The embodiments of the present invention may be implemented usingvarious means. For instance, embodiments of the present invention may beimplemented using hardware, firmware, software and/or any combinationsthereof. In case of the implementation by hardware, one embodiment ofthe present invention may be implemented by one of ASICs (applicationspecific integrated circuits), DSPs (digital signal processors), DSPDs(digital signal processing devices), PLDs (programmable logic devices),FPGAs (field programmable gate arrays), processor, controller,microcontroller, microprocessor and the like.

In case of the implementation by firmware or software, one embodiment ofthe present invention may be implemented by modules, procedures, and/orfunctions for performing the above-explained functions or operations.Software code may be stored in a memory unit and may be driven by aprocessor. The memory unit may be provided within or outside theprocessor to exchange data with the processor through the various meansknown to the public.

It will be apparent to those skilled in the art that the presentinvention can be embodied in other specific forms without departing fromthe spirit and essential characteristics of the invention. Thus, theabove embodiments are to be considered in all respects as illustrativeand not restrictive. The scope of the invention should be determined byreasonable interpretation of the appended claims and all change whichcomes within the equivalent scope of the invention are included in thescope of the invention.

INDUSTRIAL APPLICABILITY

Although an example in which a method and apparatus for transmitting andreceiving a synchronization signal for device-to-device (D2D)communication in a wireless communication system is applied to a 3GPPLTE system is described, the present invention is applicable to variouswireless communication systems in addition to a 3rd generationpartnership project long term evolution (3GPP LTE) system.

The invention claimed is:
 1. A method of acquiring synchronization fordevice-to-device (D2D) communication by a user equipment (UE) in awireless communication system, the method comprising: receiving, by theUE in coverage of a cell related to a base station (BS), information ona synchronization type for the D2D communication; acquiringsynchronization for the D2D communication using a synchronizationsource, based on the information on the synchronization type; andtransmitting information on the synchronization source used by the UE tothe BS, wherein the synchronization source is one of i) the cell relatedto the BS and ii) an external synchronization source different from theBS, and wherein, when the information on the synchronization type isrelated to the BS, the cell is used as the synchronization source, andwhen the information on the synchronization type is related to theexternal synchronization source, the external synchronization source isused as the synchronization source.
 2. The method according to claim 1,wherein, when the information on the synchronization type is related tothe BS, a synchronization for the D2D communication is acquired usingonly a synchronization signal received from the BS.
 3. The methodaccording to claim 1, wherein, when the information on thesynchronization type is related to the external synchronization source,time synchronization for the D2D communication is acquired based on asynchronization signal received from the external synchronizationsource, and frequency synchronization for the D2D communication isacquired based on a synchronization signal received from the BS.
 4. Themethod according to claim 1, further comprising: receiving informationon resource allocation from the BS, wherein the information on resourceallocation comprises information on a resource for receiving asynchronization signal from the external synchronization source.
 5. Themethod according to claim 1, wherein the D2D communication is performedaccording to synchronization acquired from the external synchronizationsource during a predetermined time interval from a specific point intime defined by the external synchronization source.
 6. The methodaccording to claim 5, wherein, when a signal for the BS and a signal fora counterpart UE of the D2D communication are simultaneously scheduledin the predetermined time interval, the signal for the counterpart UE isprioritized over the signal for the BS.
 7. The method according to claim1, further comprising: generating a D2D synchronization signal based ona sequence generation root index; and transmitting the D2Dsynchronization signal to a counterpart of the D2D communication,wherein the sequence generation root index is set to a different valuewhen the cell is used as the synchronization source and when theexternal synchronization source is used as the synchronization source.8. The method according to claim 1, further comprising: transmittingdata to a counterpart UE of the D2D communication, wherein a parameterof the data is differently set when the cell is used as thesynchronization source and when the external synchronization source isused as the synchronization source.
 9. The method according to claim 8,wherein the parameter of the data comprises at least a demodulationreference signal sequence formation parameter or a scrambling sequenceformation parameter.
 10. A user equipment (UE) for acquiringsynchronization for device-to-device (D2D) communication in a wirelesscommunication system, the UE comprising: a transceiver configured totransmit and receive a signal to and from a base station (BS), anexternal synchronization source different from the BS, or a counterpartUE of the D2D communication; and a processor connected to thetransceiver, wherein the processor is configured to: control thetransceiver to receive, from the BS, information on a synchronizationtype for the D2D communication, wherein the UE is in coverage of a cellrelated to the BS, acquire synchronization for the D2D communicationusing a synchronization source, based on the information on thesynchronization type, and control the transceiver to transmitinformation on the synchronization source used by the UE to the BS,wherein the synchronization source is one of i) the cell related to theBS and ii) an external synchronization source different from the BS, andwherein, when the information on synchronization type is related to theBS, the cell is used as the synchronization source, and when theinformation on the synchronization type is related to the externalsynchronization source, the external synchronization source is used asthe synchronization source.